The present invention relates to a heat insulating structure adapted for use in connection with the transportation of a gas having a high temperature and pressure and, more particularly, to a heat insulating structure suitable for use in double-pipe construction for transporting high temperature and pressure gas.
Conventionally, the transportation of high temperature and pressure gases such as, for example, working gas in chemical or power plant has been made by means of a pipe which is made of a material capable of withstanding the high temperature and pressure of the gas. This pipe is usually lagged with a heat insulating material so as to prevent the heat of gas from leaking outside.
On the other hand, the current tendency of diversification of energy utilization and improvement of the efficiency in these chemical and power plants require a high temperature of the working gas well reaching 1000.degree. C., which can hardly be withstood by conventional piping material especially under the presence of the high pressure of the gas. Under this circumstance, it is becoming popular to use double-pipe construction having internal heat insulating material, for transporting such a high temperature and pressure gas.
This double-pipe construction is therefore required to compensate for the reduction of mechanical strength of the pipe attributable to the temperature rise of the piping material, while fulfilling the ordinary purposes of sealing of the gas and preservation of the heat. Consequently, the performance of the heat insulating material or the heat insulating structure, as a factor in designing the construction of the pipes for transporting high temperature and gas pressure, is becoming more critical.
Most of conventionally used heat insulating materials are materials of a type so called lagging, which are adapted to be used under atmospheric pressure. In usual cases, the required heat insulating effect can be obtained by merely attaching the heat insulating material of this kind having a suitable thickness to the portion where the heat insulation is to be made. Therefore, the value of the thermal conductivity can be represented relatively easily, as a function of the mean temperature of the heat insulating material.
In good contrast to the above, in case that the heat insulating material is used in a high temperature and pressure gas such as helium having a thermal characteristic quite different from that of air, the thermal conductivity of the heat insulating material is represented as a function of various factors such as temperature, and the pressure of the gas, radiation factor, Prandtl number, Dhassi number, Rayleigh number, density of the filling of the heat insulating material and so forth. Thus, the heat insulating material exhibits extremely complexed characteristics when used in the high temperature and pressure gas.
Further, when a conventional porous heat insulating material is used in combination with the double-pipe construction for high-temperature use, the heat insulating material is soon deformed due to the sliding movement of the surface of the heat insulating material in relation to the inner surface of the pipe.
In addition, the restorability of the heat insulating material is gradually deteriorated, due to continuous operation for many hours under the presence of high temperature and pressure, and due to a repeated or cyclic change of the temperature and pressure. Consequently, gaps or vacant spaces of gradually increasing volume are formed here and there in the heat insulating structure. All of these phenomena lead to an increment of the thermal conductivity, i.e. the deterioration of the heat insulating performance.
A typical double-pipe of the kind described has a construction as stated below.
Namely, the double pipe has an outer pipe, an inner pipe disposed in the outer pipe and spaced from the latter by means of spacers, sealed pipes adapted to circulate a pressurized cooling gas of low temperature through the annular space formed between the outer and inner pipes, a dummy pipe made of a heat-resistance metallic material and disposed in the inner pipe so as to define a passage of the working gas to be transported, and heat insulating structures disposed between the dummy pipe and the inner pipe, adapted to reduce the amount of heat exchange between the working gas under transportation and the cooling gas and to reduce the temperature differential across the wall of the inner pipe.
This heat insulating structure consists of a fibrous heat insulating material and a hard felt-like shaped fibrous heat insulating material for preventing a part of the heat insulating material from being carried away by the working gas. A plurality of heat insulating structures are disposed alternatingly and independently in the axial direction. The arrangement is such that the thermal expansion of the heat insulating structure in the axial direction is conveniently absorbed by the gaps or vacant spaces preserved between the adjacent structures.
This heat insulating structure consists of two layers separated by an intermediate partition pipe having annular slits for the substitution of the gases within the heat insulating structure. The intermediate partition pipe disposed along the inner layer of the heat insulating structure is spaced by spacers from the dummy tube, so as to preserve a predetermined gas therebetween. The dummy tube is provided with a plurality of slide joints disposed in the axial direction thereof, so as to absorb the thermal expansion in the axial direction.
The conventional heat insulating structure for double pipe as described above, however, exhibits a large deformation and allows the formation of various forms of gaps, due to the sliding movement of the high temperature gas sealing pipe, intermediate partition pipe and the fibrous heat insulating material, in relation to one aother, attributable to the difference of the coefficients of the thermal expansion. Further, the restorability of the heat insulating material is gradually deteriorated, as it is subjected to the high temperature and pressure gas for many hours and to abrupt changes of pressure and temperature due to the repeated start and stop of the plant, as well as repeated change of load. Consequently, the size of the gap or vacancy is gradually increased as the time elapses, so as to deteriorate the heat insulating performance.
Particularly, when the gaps or vacancies are formed to extend in the radial direction, the gas under transportation inconveniently invades these gaps or vacancies through the clearances in the slide joints and slits and then flows through the axial gaps. Thus, the radial gaps or vacancies and the axial gaps in combination form so-called by-pass passages of the gas.
The increase of the apparent thermal conductivity of the heat insulating structure, i.e. the deterioration of the heat insulating performance of the double pipe attributable to these gaps or vacancies is much larger than that caused by natural convections in the heat insulating materials and in the gaps, so that it becomes necessary to design the outer and inner pipes to have larger inner diameters. Further, the deterioration of the heat insulating performance requires an increased flow rate of the cooling gas and, in addition, makes the temperature differential across the wall of the inner tube larger, resulting in an increased thermal stress and, accordingly, deteriorated reliability of the piping as a whole.
Further, since the heat insulating materials are not fixed, they can easily be displaced in the longitudinal direction. Therefore, in case of an earthquake or when a vibration is imparted to the pipes, the heat insulating materials tend to be moved in the longitudinal direction to make the axial gaps larger, so as to locally form large by-pass passages. In such a case, a large amount of hot gas under transportation is brought into direct contact with the inner pipe, often causing hot spots which in turn cause cracks in the inner pipe.
It is considered that the by-pass passages of the gas, which are formed, as stated before, to include the clearances in the sliding joints and slits, have highly complicated shapes and patterns of communication. However, concerning the layer of lower temperature of the heat insulating structure, these by-pass passages can be sorted into the following three kinds of passages, namely passages A extending along the intermediate partition pipe, passages B formed axially at the center of the heat insulating materials and passages C formed along the sealed inner pipe.
The rate of increase of the thermal conductivity, i.e. the ratio of the thermal conductivity exhibited by the heat insulating structure after the by-pass passages are formed to the same thermal conductivity before the by-pass passages are formed, sharply increases as the temperature of the gas transported becomes higher and the positions of the by-pass passages become closer to the low temperature side. At the same time, the rate of increase of the thermal conductivity becomes larger, as the pressure of the transported gas increases, because the thermal conductivity of the gas itself is increased as the pressure becomes higher. In addition, needless to say, the rate of increase of the thermal conductivity becomes larger as the flow rate of the gas flowing through the by-pass passage becomes larger.
At page 23 of the Study of Machines, vol. 26, No. 10 (1974), it is suggested to fix the heat insulating material disposed in a pressure-resistant pipe, i.e. the sealed pipe, to the wall of the pressure resistant pipe, thereby to prevent the formation of the by-pass passages. This way of solution is however still insufficient because the restorability of the heat insulating material is gradually deteriorated, as stated before, due to the repeated generation of thermal stress attributable to the difference of the coefficients of thermal expansion between the pipe and the heat insulating material, so as to cause various gaps which soon grow to form by-pass passages.
As has been explained, in the conventional pipes for transporting the gases of high temperature and passure, the deterioration of the heat insulating performance of the heat insulating material attributable to the formation of by-pass passages is considerably large. This tendency becomes more serious as the temperature and/or the pressure become higher.
All other constructions containing therein a gas of high temperature and pressure and having a heat insulating construction suffer the same problem.
Thus, how to prevent the formation of gaps in the heat insulating material has become a key to the safe and efficient handling of the working gases in various plants, the pressure and temperature of which are becoming higher recently.